1. Field of the Invention
The invention relates to optical fibers and, more particularly, to optical fibers which have a high tensile strength and can thus withstand a substantial amount of stress.
2. Description of the Prior Art
Often instrument packages are suspended by cables and lowered down vertical passages, such as, for example, oil or gas well-heads. As an instrument package is lowered down the well, it maps the existence and depth location of various geological phenomena, such as mineral bearing strata. Power for the instruments and signals emanating from the instruments are transmitted through cables to various electronic systems, e.g., control electronics and recorders, all located on the surface. By virtue of the small diameter of the well hole, the diameter of the cable must be kept small. In addition, the cable must be able to withstand the high tensile load imparted to it by the suspended instrument package.
As noted in, B. A. Proctor et al., "The Strength of Fused Silica," Proc. R. Soc. London, Series A 297, pages 534-557, glass fibers are among the strongest known materials with theoretical tensile strengths approaching 2.times.10.sup.6 psi. Presently existing glass fibers are available in diameters ranging from 0.125 mm to 0.250 mm. By virtue of the small resulting cross-sectional size, a single glass fiber is disadvantageously incapable of sustaining a large tensile load.
Consequently, a cable which can withstand a moderate level of stress typically contains a number of these fibers intertwined with an array of separate strength members, such as steel wire or any one of several well known non-metallic members such as Kevlar.TM. (a trademark owned by E. I. duPont de Nemours and Co.) brand yarn manufactured by DuPont. However, for a number of reasons, such cables are often inadequate in the above-mentioned application of suspending an instrument package and lowering it down an oil or gas well-head. Specifically, as the cable is designed to withstand increasingly larger amounts of stress necessitated by supporting increasingly heavier instrument packages, the diameter and/or number of necessary strength members used in the cable disadvantageously increases. Consequently, such a cable containing both glass fibers and strength members often becomes excessively thick and quite unwieldy.